The Amino Group

Transcript

0:00Introduction

Hello everyone! Let us start with a strange lab test. Usually we look for a colour or a gas, but in this test the positive result is a horrible smell. You warm a compound with chloroform and alcoholic potash, and the whole lab starts to stink. So what is that smell telling you? By the end of this lesson you will know. These tests come up often in JEE practical chemistry, so let's begin.

0:27Test 1: solubility and litmus

So, first meet our group. On screen is the amino group, N H two; the little dash is its bond to the rest of the molecule, and the small two written low counts the hydrogens. Capital R is a carbon group. One R on the nitrogen, RNH two, means primary; two, R two N H, mean secondary; and three, R three N, mean tertiary. Every test today depends on this, okay? Test number one is solubility. An amine that does not dissolve in water dissolves in dilute hydrochloric acid, H C l, because it forms a salt with the acid. Right? Next comes litmus. Aliphatic amines, with only alkyl groups on the nitrogen, turn red litmus blue. Now, aniline has its nitrogen bonded directly to a benzene ring. Will aniline turn red litmus blue too? If you said yes, careful, that is the trap. The lone pair on its nitrogen is delocalised into the ring, so aniline is too weak a base to affect litmus. That is why the rule says aliphatic amines only. Clear?

1:34Test 2: the carbylamine (isocyanide) test

Test number two is the carbylamine, or isocyanide, test. Warm the amine with chloroform, CHC l three, and alcoholic KOH, potassium hydroxide in alcohol; potash, in brackets, is its other name. With me? Here is the equation; the arrow means gives. RNH two, plus chloroform, plus three KOH, gives RNC, three K C l and three water molecules. So one mole of amine uses three moles of KOH, okay? Now, there is no colour change here. So how do you know it worked? You know it by the smell. The product, RNC, is an isocyanide with a really foul smell. Now tell me, will a secondary amine give this smell too? It will not, because only primary amines give this test, aliphatic and aromatic alike; secondary and tertiary amines do not. So that answers our opening puzzle: the horrible smell means a primary amino group. Got it?

2:37Test 3: the azo dye test

Test number three is the azo dye test, only for aromatic primary amines, with the nitrogen directly on a benzene ring. The amine is diazotised with sodium nitrite, N a N O two, and H C l; the slash means together. The temperature is zero to five degrees Celsius; the little circle means degrees. Why must it be so cold? It is kept in ice because the diazonium salt formed is unstable. Above about five degrees Celsius, it decomposes to phenol and nitrogen gas, N two. Okay? Then the cold solution goes into alkaline two naphthol, that is, two naphthol made basic, and an orange-red dye forms. Now here is a trap. Benzylamine is primary, and it has a benzene ring too. Will it give the dye? Benzylamine gives no dye at all. Its nitrogen is not on the ring, so it behaves as an aliphatic amine and gives nitrogen gas with nitrous acid. Aniline gives the dye, so this test tells the two apart. Right?

3:44Test 4: nitrous acid, HNO₂

Test number four uses nitrous acid, HNO two, and each class gives a different result. Aliphatic primary amines give nitrogen gas, seen as effervescence, which means fizzing. Example two is built on this. Clear? Secondary amines give a yellow oily nitroso amine, so a yellow oil separates. Okay? And tertiary aliphatic amines just dissolve as salts. So remember the pattern: bubbles, a yellow oil, or simply dissolving. Got it?

4:15Test 5: Hinsberg's test

Test number five is Hinsberg's test. The reagent is benzene sulphonyl chloride, with alkali. Now look at the alkali. Of the primary and the secondary amine, only one gives a product that dissolves in it. Which one is it? It is the primary amine's product that dissolves in alkali. This pair is easy to swap, so remember that primary means soluble. With me? Secondary amines give an insoluble product, and tertiary amines do not react at all. One reagent sorts all three classes. Nice, isn't it?

4:49Example 1: which amines give the carbylamine test

Time for Example one. Which of these give the carbylamine test: aniline, N methyl aniline, ethylamine, triethylamine and benzylamine? Here is a hint: count the carbon groups on each nitrogen. Pause the video and try it first. Done? Okay, let's check. Aniline and ethylamine each have one carbon group on the nitrogen, so both are primary and give the test. Now, N methyl aniline has aniline in its name, so it is primary too, right? If you said yes, that is exactly the trap. The capital N in front of methyl means the methyl sits on the nitrogen itself. So the nitrogen has two carbon groups, the ring and the methyl, which makes it secondary, with no carbylamine test, okay? Triethylamine has three ethyl groups on the nitrogen, so it is tertiary, and it does not give the test either. Right? Now look at benzylamine. Its formula on screen is a benzene ring, C six H five, then a C H two group carrying the N H two. So the nitrogen holds just one carbon group, which makes it primary, and it gives the test. Clear? So the answer is aniline, ethylamine and benzylamine. These three give the horrible smell from our opening puzzle. Got it?

6:11Example 2: volume of N2 from 0.59 g of propan-1-amine

Now a harder numerical. What volume of nitrogen at STP is evolved when zero point five nine grams of propan one amine, molar mass fifty nine, is treated with excess nitrous acid? STP is standard temperature and pressure, and capital M is the molar mass. Pause the video and try it first. Done? Okay, let's check. Propan one amine is a chain of three carbons with the N H two at the end, as on screen. One alkyl group on the nitrogen, so it is aliphatic primary, and from test four it gives nitrogen gas. With me? Small n, the moles of amine, is zero point five nine over fifty nine, which gives zero point zero one zero; m o l means mole. Now, the amine has one nitrogen atom, but N two has two. So only zero point zero zero five moles of nitrogen, right? That is wrong, and this is the trap! Look at the equation: RNH two plus nitrous acid gives the alcohol, ROH, plus N two, plus water. One nitrogen atom comes from the amine and the other from the nitrous acid. Okay? So one mole of amine gives one mole of N two, and here that means zero point zero one zero moles of nitrogen. Right? At STP, one mole of gas occupies twenty two thousand four hundred millilitres, m L on screen. So the volume, capital V, is zero point zero one zero times twenty two thousand four hundred, which gives two hundred and twenty four millilitres. The trap would have given only half.

7:51Quick revision: carbylamine, azo dye, HNO2, Hinsberg

Got it? So, quick revision. The carbylamine test is for primary amines only, and gives a foul-smelling isocyanide. The azo dye test needs an aromatic primary amine at zero to five degrees Celsius, and gives an orange-red dye with two naphthol. With nitrous acid, aliphatic primary amines give nitrogen, secondary amines a yellow oil, and tertiary amines dissolve. In Hinsberg's test, the primary product dissolves in alkali, the secondary is insoluble, and tertiary amines do not react. Now try the practice problems on this page. All the best!